During the production of the well, it is known to measure the flow rate of fluid extracted from the well to be able to monitor the quantity and quality of the production. In particular, it is frequently necessary for the operator of the well to determine the overall flow rate of fluid flowing through the pipe, and if possible, the individual volume flow rates of each phase flowing in the pipe.
To determine these values, it is necessary to estimate the volume fraction of gas contained in the multiphase fluid, often referred to as “gas hold up,” and the proportion of aqueous phase present in the liquid at any moment.
To estimate these parameters, it is known to determine the relative areas occupied by each of the gas phase, the oily liquid phase and the aqueous liquid phase on a section of the pipe. To that end, a radioactive source is placed opposite a wall of the pipe to emit gamma photons at typically one or more energy levels.
The gamma photons are then oriented transversely through the fluid flowing in the pipe. A detector is placed opposite the source, opposite the pipe to collect and count the photons that pass through the multiphase fluid and determine their energy. The count number received at each energy is measured at a high frequency, for example with a sampling pitch in the vicinity of 20 ms. This makes it possible to calculate the lineic fraction of the gas, i.e. the ratio between the length of the gaseous phase crossed and the internal diameter of the pipe.
Due to the nature of the radioactive source, there is a natural statistical dispersion of the number of counts measured, even in the case where the measured fluid is static. The measurement uncertainty resulting from this dispersion can be reduced significantly by increasing the integration time.
However, in practice, the multiphase fluid circulates in the pipe at a high flow rate. This fluid is generally turbulent and sometimes has structural irregularities, for example gas bubbles in liquid, or plugs in gas that make the flow unstable.
Measuring the number of gamma photon counts remains an effective way to measure the phase fractions, regardless of the state of flow. However, in a dynamic state, the number of counts cannot be significantly averaged to reduce the statistical noise, since the nature of the flow can vary quite rapidly.
Subsequently, in certain cases, the results obtained directly based on count measurements done in a dynamic state have significant fluctuations.
To resolve this problem, U.S. Pat. No. 5,854,820 proposes a method in which the statistical fluctuations of the measured numbers of counts are compensated by an algorithm making it possible to model the attenuations and compare them to the measured attenuation. A statistical residual is calculated and is minimized to estimate the fractions.
Such a calculation increases the precision of the measurement, but can still be improved, in particular to optimize the determined values of the gas content levels and the proportion of water, in particular when these values are close to their physical boundary values.
One aim of the present disclosure is therefore to provide a method for calculating phase fractions that are more precise, even when the values of the measured fractions are close to their physical boundaries and/or it is not possible to significantly average the measured numbers of counts, given the dynamic nature of the flow.